Duchenne Muscular Dystrophy Gene Therapy
About Duchenne muscular dystrophy is the most common and life threatening form of muscular dystrophy. This disorder is generally associated with muscle degeneration, weakness, and ultimately mortality due to the loss in ability of skeletal muscles to contract. DMD is a genetically inherited X-linked trait that is caused by a mutation in the cytoskeletal protein dystrophyin. Dystrophin is important in strengthening muscle fibers and protecting them from injury during the phases of muscle contraction and relaxation. The dystrophin complex acts as an anchor, connecting the muscle cell's structural framework with a lattice of proteins and extracellular molecules. It may also play a role in chemical signaling, as dystrophin is associated with various signalling molecules involved in muscle contraction. As a result, a mutated dystrophin gene as seen in DMD is a potentially fatal disorder leading to skeletal muscle failure. Methods Due to that fact the DMD is caused by monogenic mutations in the dystrophin gene, the disorder is thought to be treatable through gene therapy. It has been shown that recombinant adeno-associated virus (rAAV)vectors have the potential to reverse muscle failure. These virus vectors can carry a miniaturized functional dystrophin gene. Termed minidistrophin, the gene is expressed after AAV delivery into dystrophin deficient organisms. Expression is shown to correctly localize to the sarcolemma, restore the missing dystrophin associated protein complex to the cell membrane, and improve the dystrophic pathology of skeletal muscle. The AVV viruses are used in such popularity for this purpose because they can infect non-dividing cells, and their genomes are expressed for an extended period of time. The AAV genome is built of single stranded DNA, about 4.7Kb in length. The genome comprises inverted terminal repeats at both ends of the DNA strand, and two open reading frames. These two reading frames create a transgene expression cassette replacing original rep and cap genes. Co infection with a helper virus is required for replication of AVV and productive infection. This allows for the minidistrophin gene to be taken up by the virus, and therefore as it infects a cell, it incorporates it's geneome resulting in an effective substitution for the mutated distrophin gene. Immunological Hurdles Infection with viruses as a means of gene therapy has the potential to elicit an immune response. Innate or acquired immune defense mechanisms can block or limit the extent of gene transfer and cause destruction of the cells undergoing genetic therapy. Problems from innate immune responses are mainly observed with intravenous injections of high doses. Acquired immunity which involves cellular responses activated through B and T cell mediated pathways pose a risk of antibody mediated reactions against vector or transgene-encoded proteins. The antibodies can block vector delivery to the muscle cell, however do not pose a threat to long term expression for an intramuscular protein. Immune suppression therapies may be considered as a way of avoiding immunological counteractions with AAV gene therapy, however this comes with a significant risk of increased infection. It is important to weigh the benefits of the therapy with the associated risks when considering AAV as an effective treatment in a clinical setting. Resources. 1. [http://www.genetherapynet.com/viral-vectors/adeno-associated-viruses.html "Gene Therapy Adeno-Associated Virus (AAV) Vectors Explained." Gene Therapy Adeno-Associated Virus (AAV) Vectors Explained. N.p., n.d. Web. 12 Oct. 2014.] [http://www.nature.com/mt/journal/v20/n2/full/mt2011237a.html 2. Bowles, Dawn E. "Phase 1 Gene Therapy for Duchenne Muscular Dystrophy Using a Translational Optimized AAV Vector." Nature.com. Nature Publishing Group, n.d. Web. 12 Oct. 2014.] 3. [http://hmg.oxfordjournals.org/content/11/20/2355.full#ref-54 "Human Molecular Genetics." Gene Therapy of Muscular Dystrophy. Oxford Journals, n.d. Web. 12 Oct. 2014.]